This invention relates to phased array antennas and, more particularly, to a system for forming a beam of radiation at various frequencies of radiation.
Arrays of radiating elements are utilized for forming beams of radiant energy for both electromagnetic energy and sonic energy. In the case of sonic energy, the beams are generally formed by transducers of a sonar system, In the case of electromagnetic energy, the radiating elements may take the form of dipoles or other form of radiating elements. In both the cases of electromagnetic and sonic energies, beam-steering units form the beam and direct the beam by the control of delay or phase shift of the radiant energy from one radiating element relative to the radiant energy from a second radiating element of the array. The beam may be made to scan across a region of space, or may be made to jump from region to region as in the case of the tracking of targets located in different directions from the antenna.
While the invention is useful in all of the foregoing situations, it is most readily described for the case of a scanning antenna radiating electromagnetic energy as in the case of a phased-array antenna of a microwave landing system for aircraft at an airport. Therein, a beam scans back and forth to both sides of a runway for use by an incoming aircraft in the generation of guidance signals which guide the aircraft to the runway. Typically, such a beam would be scanned approximately 30.degree. to either side of the runway.
A problem arises in that the beam-steering unit is designed to produce a beam at a specific frequency of electro-magnetic energy. However, in the foregoing in microwave landing system (MLS), it is desirable that the beam-forming be accomplished over a range of frequencies so as to accommodate different signal channels, each characterized by its own frequency, for use by respective ones of the incoming aircraft.
One attempt at solution of the foregoing problem is the utilization of beam-steering units which have been adapted to form beams at each of a number of frequencies. Typically, a beam-steering unit includes a memory for storing data as to the requisite phase shift where phase shifters are utilized, or delay where delay units are utilized, for each radiating element for each direction in which the beam is to be pointed relative to the antenna array. In the case of a scanning antenna, many incremental steps in direction are provided, with each step being less than a beamwidth, so that the beam appears to be smoothly scanned through space even though it is, in fact, being scanned by a rapid succession of steps in direction. The foregoing storage of phase data or delay data would be repeated for a second frequency and for a third frequency, and again for still further frequencies, in the case where the beams are to be formed at different frequencies of radiation. Thereby, the beam-steering unit is able to form and steer the beams at different frequencies of radiation.
The foregoing solution to the problem is disadvantageous in that it requires far more storage than would be required for the single frequency case. The disadvantage is manifested both in terms of system cost and system complexity. In the case of an MLS wherein redundant circuits may be utilized to obtain high reliability, the disadvantage of the utilization of additional memory becomes magnified.